CA1161574A - Target illumination with multiple pass free electron laser - Google Patents

Abstract

Target illumination wherein a free electron laser is disposed in an optical resonant cavity having an optical focus spaced from the amplifier, the amplifier is energized by periodic bursts of electron energy, with the period between such bursts being coordinated with the overall length of the optical resonant cavity such that optical pulses stimulated in the laser are cumulatively additive. A target to be illuminated is injected at the focal point in the resonant cavity after the cumulatively additive pulse has reached preselected energy.

Description

RAC/RCC/gm ~ 4 Target Illumination With Multiple Pass Free_Electron_Laser The present invention pertains to target illumination, and more particularly to techniques for focusing high-ener~y pulses of electromagnetic radiation onto a target focal region.
Hig~-ener~y pulsed laser systems have been developed for irradiating minute targets. Such systems generally include a laser oscillator for generating a train of optical pulses and a series of laser amplifiers.
A single pulse from the oscillator output train is sèlected using electro-optical switches, and the selected pulse is then passed to the laser amplifiers.
- One or more converging high intensity pulses of electro-magnetic radiation may be produced, and a series of lenses and/or mirrors may be employed to focus the pulse or pulses onto a target at the target region.
~ditional elements such as polarizers, Faraday rotators and Pockels cells are often provided for controlled routing of the illumination beams.
One problem presented in the various prior target illumination systems which include refractive transparent elements, such as lenses and laser slabs or rods, lies in the fact that the indexes of refraction of such elements are non-linear functions o beam intensity. These non-linearities cause sel~-focusing of the high-power optical beams, and can lead to damage of all optical components and loss of focusable beam energy. Thermal effects and distortions in transparent optical components may also limit the repetition rate '~;

1.

~fil57~

of prior ar~ laser systems. Another problem is that the index of refraction and focal properties of the transparent elements vary with frequency, which means that substantial redesign and rework is required if the illumination frequency is to be modified.
Objects of the present invention are to provide systems and methods ~or generating one or more beams oF electromagnetic energy and for ocusing such energy onto a target region, which systems and methods possess enhanced efficiency, in which the frequency of the illumination beam or beams may be readily adjusted without substantial alteration of system components, which may operate entirely in a vacuum to reduce distortions, and/or which eliminate the need - 15 for transparent optical elements and thereby reduce the problem of beam self-focusing and consequent potential damage to all system elements.
A further and more specific object of the invention is to provide a system and method for operating a free electron laser in a pulsed mode, and for thereby generating high powered optical pulses and focusing such pulses onto a target region. ~~
These and other objects are accomplished in accordance with the invention by providing an optical resonator or cavity comprising an amplifier disposed in a bi-directional beam path defined by a plurality of highly reflective mirrors having a focal point for target illumination spaced from the amplifier. In a preferred embodiment of the invention, the amplifier comprises a free electron laser amplifier disposed to receive stimulating pulsed electron energy and for emitting corresponding pulses of optical energy. The length of the cavity beam path and the period of the pulsed electron stimulation are coordinated such that the previously emitted optical pulses and the electron pulses pass through the-amplifier simulatneously.
Thus, successively generated optical pulses are cumulatively additive in the laser amplifier. A target is injected into the target focal region when the cumulative optical beam has reached a desired power level.
The invention so described may be disposed entirely within a vacuum. No optically transparent or refract~ve elements are required. Moreover, free-electron lasers are readily frequency-tunable in the visible, X-ray, infrared and ultraviolet regions, so that this invention could be used over a wide range of frequencies.
The invention, together with additional objects, features and advantages thereof, is more fully set forth in the following description, the appended claims and the accompanying drawings in which:
FIG. 1 is a schematic illustration of a prior art free electron laser system;
FIG. 2 is a schematic illustration of a basic embodiment of the target illumination system provided by the present invention, FIG. 3 is a graph useful for illustrating operation of the embodiment of FIG. 2; and FIGS. 4-8 are schematic illustrations of respective alternative embodiments of the invention.

Before proceeding with a detailed description of several embodimen-ts of the invention, it will be appreciated that the various elements - e.g. mirrors, magnets, electron accelerators, etc. - are illustrated schematically in the drawing figures. Suitable structure for each of such elements, where not described in detail herein, will be self-evident to the skilled artisan. Similarly, suitable means for mounting such structural elements relative to each other as described herein will be manifest. The preferred embodiments of the invention comprise a free electron laser, one prior art type of which will be described in connection with FIG. l.
As noted above, the free electron laser is frequency tunable in the infr~red, visible, ultraviolet and X-ray frequency ranges. Thus the terms "optic" and "laser" as used herein are not limited to light in the visible region, but encompass at least the other electromagnetic frequency regions noted above.
FIG. 1 illustrates a free electron laser system similar to that disclosed in U. S. Patent No. 3,822,410 issued July 2, 1974 to John M. Madey which produces a continuous series or train of low power laser pulses. A plurality of periodic magnets 20, which constitute the laser amplifier, are disposed in a linear array along the optical axis 22. A
continuous electron stream is routed by opposed bending magnets 24,26 in a closed loop 28 which includes opposed substantially linear loop portions 30,32. Loop portion 30 is coaxial with that portion of beam axis 22 which intersects magnets 20. An electron accelerator cavity 34 is disposed in linear path portion 32 and is coupled to an RF generator 36 for maintaining ~3 7 ~

circulating electron energy at a desired level. Passage of electrons through magnets 20 stimulates emission of photon energy in a colLimated beam at a freque~cy which is a function of elec-tron energy, spacing of magnets 20 and magnet fiela strength. Partially transparent mirrors 38 permit a small fraction of the incident laser light to pass and thereby produce a train or series of optical pulses as previously described.
FIG. 2 illustrates a basic embodiment of the present invention as comprising a concentric optical resonator or cavity defined by a pair of opposed con-cave mirrors 15,16 having a common focal point 17 on the bi-directional beam path 19 therebetween. Con-cerning resonant cavities generally, reference may be had to Yariv, Quantum Electronics, Wiley ~ Sons, 2nd Ed., Ch. 7 (1975). ~ free electron laser amplifier 14 is disposed on beam path 19 spaced to one side of focus 17. An electron accelerator 10 produces a periodic series of electron pulses, as opposed to a continuous electron stream, one such electron pulse being indicated at 12. Pulses 12 are guided along a~
trajectory 13 by suitable bending magnets (not shown) and are thereby fed through amplifier 14 coaxially with beam path 19. Periodic passage of electron pulses 12 through amplifier 14 stimulates emission of pulsed photon energy in a collimated beam, which pulsed energy then resonates between mirrors 15,16 and is focused on each passage at 17. The length of beam path 19 is coordinated with the repetition period of electron pulses 12 such that successive bursts of photon radiation 5 ~ ~

in amplifier 14 are cumulatively additive to previous pulsed emissions, the pulsed laser beam 11 traveling on path 19 being amplified generally stepwise on successive passes through amplifier 14 as indicated schematically at lla-llc.
The electron pulse emission frequency (f) of accelerator 10 in FIG. 2 is givèn by the equation c f = 2d tl) where e is the speed of light in the vacuum in which the cavity is disposed, and d is the cavity length.
For d equal to 300 meters, f would be equal -to 0.5 MHz. A linear induction accelerator triggered using vacuum tube switches could be made to operate at this repetition rate.
In general, the amount of amplification in amplifier 14 on successive passes of the optical beam will vary as a function of beam intensity and/or amplifier magnet design. Beam collimation improves after a number of passes through the amplifier and, for passes on the order of a few tens or more, optical losses (primarily at mirrors 15,16) become substantially constant, If it is assumed that the fraction of optical energy lost on each pass through amplifier 14 is con-stant, which is approximately true after a number of 3 ~

passes as previously noted, then beam power P in the optical cavity after n passes of the laser beam through amplifier 14 will be given by the equation P ~ Q Pi (l-~)n-i (2) w~ere a Pi is the incremental power increase in amplifier 14 on the ith pass and ~is the fractional energy loss pe~ pass. If it is further assumed that the increase in optical power per pass a P is constant for all passes, equation 1 simplifies to p = ~ )n). (3) 6a.

11~6~7~

Equation 3 is illustrated graphically at 21 in FIG. 3. Cuxve 21 demonstrates that output laser power P approaches an asympotitic limit 23 e~ual to ~ P/~ after a number of passes n at which the optical gain per pass equals the losses per pass. Graph 25 illustrates the total energy generated in lase~
whereas curve 21 illustrates the actual energy available in the optical cavity, i.e. total energy minus losses.
At n = l/S , available energy is equal to substantially 63% of total potential energy. Using high quality dielectric mirrors with coefficients of reflectivity on the order of 0.9995, round trip optical losses of no more than a few tenths of a percent per pass are incurred.
When the number of optical passes n is chosen equal to 1/~ , power P in the optical cavity of FIG. 2 is related to electron current I by the expression P - 0.63 I~EN (4) where~ is the average fractional energy in electron beam 12 converted to optical energy per pass, E is the electron energy and N is the number of amplifier passes per target shot. In the single-amplifier embodiment of FIG. 2 with a single optical pulse 11 in the optical resonator, N = n. By way of example, for beam power P of one gigawatt at focus 17 utilizing one hundred passes N, electron energy E of 500 MeV and a laser efficiency ~ of one percent, an electron current I of 3.17 amps ~ould be required, Similarly, for a beam power of one terawatt utilizing three hundred passes N, for the same electron energy and an amplifier having a two percent extraction efficiency 3 3 ~5'~

7~, a peak electron current of 529 amps would be required.
This peak electron current could be reduced by reducing the optical losses and thereby permitting a larger number of optical passes per target shot, by utilizing more than one laser amplifier (FIGS. 7 and 8 to be described), by providing two optical pulses in the cavity per target shot (FIG. 5),by utilizing more than one optical cavity (FIGS. 6 and 7) or by increasing the fractional energy extraction per pass ~ , In this connection, it should be noted at this time that each of the foregoing equations (1)-(4) will be eq~ally applicable (with an appropriate multiplication factor) to the embodiments yet to be described.
~ 15 After a number of passes calculated in the manner previously described to yield an illumination pulse of desired intensity, a target is propelled by an accelerator 18 toward focal point 17 to arrive at the focal point coincidentally with the focused beam~
The target may comprise, for example, a glass sphere or shell on the order of 1 mm in diame-ter filled or impregnated with deuterium-tritium fuel in a preferred application of the present invention to inertial con-finement fusion research. The target must be fired at a velocity which is sufficiently high that the target will intercept the illumination beam on the intended pass, but does not intercept the beam on the preceeding pass. For a round trip optical path 19 of a few hun~red meters in length, for example, a target velocity on the order of 105cm/sec. is contemplated.
Accelerator 18 may comprise a magnetic accelerator, and ~ .

7 ~

the -target may be coated with or carried by a material for conducting surface currents induced by the magnetic acceleration field. Suitable -timing means (not shown) couple electron accelerator 10 to target accelerator 18 for temporal coordination of beam generation and target`injection.
In the embodimen-t of FIG. 2, the pulsed electron energy from accelerator 10, ater passage through amplifier 14, is preferably fed to suitable means (not shown) for converting a portion of the total electron energy not extracted by the amplifier into an otherwise useful energy form. FIG. 4 illustrates an alternative em~odiment of the invention in which the pulsed electron stream is recirculated in a closed lS path 28 which includes laser amplifier 14 and an electron beam storage ring generally indicated at 41.
A magnetic switch 42 permits electron pulses to be fed into storage ring 41 from the source accelerator lOo A series of suitably timed low current pulses from accelerator 10 can be used to build up a single high current electron pulse in storage ring 41. A
pair of magnetic switches 46,48 are disposed diametrically in ring 41 se~ectively to route the pulsed electron stream in the ring onto loop 28, which extends through amplifier 14 and a booster accelerator 34.
Amplifier 14 is disposed in an op-tical resonant cavity 50 bounded by end mirrors 52,56. The curvature of mirrors 52,56, and that of an intermediate mirror 54, are such that a target focal spot 58 is produced in cavity 50 between mirrors 54,56. Amplifier 14 is disposed between mirrors 52,54. When the round trip 1 J ~ ~ ~7 ~

transit time of an electron pulse in ring 41 and loop 28 is equal to the round trip time of the laser pulse in cavity 50, a laser pulse and an electron pulse will pass through the amplifier simultaneously as pre~iously described, with the electron stream trans~
ferring energy cumulati~ely to the laser beam on each pass. When laser beam power has reached a desired level, as previously discussed in connection with equations (2)-(4), a target is injected to inter-cept the laser beam at focal point 58. Suitabletiming means lnot shown) coordinate switches 46,48 with target injector 18.
Closed loop electron paths of the type illustrated in FIG. 4 are preferred in accordance with ~ 15 the invention over open-type paths of the type illus-trated in FIG. 2. Ho~ever, the extraction of energy from the electron pulses on each pass through the laser amplifier tends to cause a spread in the energies of the individual electrons on each pass, which in turn will effect amplifier gain on succeeding passes.
In the open loop system of FIG. 2, exciting electrons are only passed to amplifier 14 one time, and hence electron spread on succeeding passes is not a problem.
Amplifier 14 in FIG. 2 may comprise a simple linear periodic magnet array of the typ shown in the above-referenced U. S. Patent 3,822,410, or may comprise a simple periodic magnet helical array of the type described in PhYsical Review Letters, Vol. 36, page 717 (1976).
In the closed loop system of FIG. 4, however, account should be taken of electron spread for peak efficiency.

1~ ~

One technique is to design amplifier 14 to have reduced sensitivity to electron spread as is discussed in Smith et al, Stanford University Report No. HEPL 830, August 1978. Another technique is to provide a modifiea 5 amplifier such as a multiple stage ampli-Eier to extract energy from the recirculating pulsed electron stream while minimizing electron energy spread. Either or both of such techniques may be utilized withou-t departing from the scope of the invention.
FIG. 5 illustrates ~7et another modified embodiment of the invention wherein two optical pulses are produced in a cavity 79 using a single recirculating electron pulse. The resonant cavity 79 is defined by a pair of opposed coa~{ial concentric mirrors 80,82 15 having a common focal point 84 at the target region.
In the modification of FIG. 5, the round trip transit time of electrons in loop 28 is equal to one-half of the round trip transit time of an optical pulse between mirrors 80,82. Thus, two high energy optical pulses 20 are produced in cavity 79 which intersect at target ~ocus 84 on every pass. The target injected at 84 thus will be illuminated by e~ual intensity beams from opposite directions.
FIG. 6 illustrates a modi~ication to the 25 err~bodiment of FIG. 5 wherein a second concentric resonant cavity 79a is defined by the opposed concave mirrors 80a,82a and has a target focus at 84 coincident with the :Eocu5 of mirrors 80,82~ A second free electron laser amplifier 14a is disposed in cavity 7ga. The 30 closed loop electron path 28a is e~panded to excite both amplifiers 14 and 14a to illuminate a target at 84 7 ~

uniformly in four quadrants. It will be understood that an electron storage ring and switches (40,~8 in FIG. 5) will be provided but are not shown in FIG.
6. AS a modification, amplifiers 14,14a may be energized by separate coordinated electron loops coupled to common or separate electron storage rings.
FIG. 7 illustrates a further modification wherein free electron amplifiers 14a-l~b are disposed in respective legs of the beam paths defined by mirrors 80,82,80a and 82a, one between each of such mirrors and the common focus 84. Each of the amplifiers is fed by an associated secondarv electron loop 28a-28d which are respectively connected to a primary closed loop 86 by the bi-directional magnetic switches 46a-46d.
~ 15 Each secondary electron loop includes an associated accelerator 34a-34d~ Additional accelerators 88 are provided in primary loop 86. FIG. ~ illustrates a further modification of the invention wherein a folded resonant cavity is defined at either end by the con-cave reflectors 90,9~. A pair of intermediate concave re~lectors 94,g6 are disposed coaxially with respective end reflectors 90,92 and are directed toward each other to form a common focal spot 98. Additionally, mirror pairs 90,94 and 92,96 cooperate to form regions therebetween of reduced beam diameter and in which a pair of free electron laser amplifiers 14a,14b are disposed.
Although the present invention has been : described in detail in connection with several alternative embodiments thereof, it will be appreciated that the invention is susceptible to any number of additional modifications and variations. For example, it is not essential for the present invention for the free electron laser ampli~iers illustrated in the various drawing FIGS. to comprise linear or helical arrays of magnets. As a modification, such magnets may be replaced by means for generating electromagnetic pump fields of suitable geometry. Similarly, in accordance with the present invention in its broadest application, the lasing means may comprise other than a free electron laser. For example, dual gas lasers have been constructed comprising a first gas for ab-sorbing outside energy and assuming a metastable state having a relatively long lifetime and a second gas for absorbing a fraction of the energy stored in -- 15 the first gas and actually performing the lasing ~unction.
The various multiple pass optical resonant cavity con-figurations illustrated in FIGS. 2 and 4-8 may be used with such dual gas lasing means for allowing enhanced energy extraction from the energy storing first gas.
Thus, in accordance with the present invention in its broa~est aspects, an optical resonant cavity is provided with at least one sharp optical focus at a target region. Lasing means are disposed in the resonant cavity for cumulatively pumping optical energy into a pulse resonating in the cavity. M~ans are provided for injecting a target into the target region to be illuminated by the cumulative optical pulse after such pulse has attained a desired energy level.
T~e invention claimed is:

13~

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1.
Apparatus for illuminating a target region with focused electromagnetic energy comprising a free electron laser including means for directing a pulsed electron stream along an electron path and lasing means disposed in said electron path and responsive to passage of a said pulsed electron stream therethrough for generating a pulsed beam of electromagnetic energy, and reflective means defining an optical resonant cavity having a beam path which includes said lasing means, said reflective means including means for focusing a said pulsed beam traveling on said beam path at a point in said beam path spaced from said lasing means to define said target region.

2.
Apparatus for illuminating a target at a target region with focused electromagnetic energy comprising a free electron laser including means for periodically directing an electron stream in a closed loop and lasing means disposed in said closed loop and responsive to each periodic passage of said electron stream therethrough for generating a corresponding beam of pulsed electromagnetic energy, reflective means disposed to define an optical resonant cavity which includes said lasing means, said reflective means being spaced such that each energy pulse generated by said lasing means is cumulatively additive to electro-magnetic energy pulses previously generated and including means for focusing said cumulatively additive pulsed beam at a point in said cavity spaced from said lasing means, and means for injecting a target into said cavity at said point when said cumulatively additive pulsed beam has reached a desired intensity.
3.

A target illumination system comprising an optical resonant cavity having a bi-directional beam path of pre-selected path length and including reflective means focusing optical energy in said cavity at a target position on said path, lasing means disposed in said cavity on said path for generating optical energy to resonate in said cavity, target injection means disposed to propel a target into said cavity at said target position such that optical energy resonating in said cavity is focused by said reflective means onto said target, and means for transferring pulsed energy to said lasing means at a frequency corresponding to said path length such that pulsed optical energy generated by said lasing means is cumulatively additive to pulsed optical energy previously generated and resonating in said cavity.

4.
The system set forth in claim 3 wherein said lasing means comprises a free electron laser.

5.
The system set forth in claim 3 wherein said lasing means comprises a dual-gas laser.

15.

6.
The system set forth in claim 3 wherein said target position is substantially centrally located in said beam path, and wherein said pulsing frequency is coordinated with said path lengths such that two cumulatively additive beams travel on said path to illuminate a said target at said target position from opposite directions.

7.
The system set forth in claim 3 or 6 further comprising second lasing means and a second optical resonant cavity with a second focus disposed to coincide with said focus of the first cavity to illuminate a target at said target position from multiple directions.

8.
A method of illuminating a target with focused electromagnetic radiation comprising the steps of: (a) providing an optical resonant cavity having a bi-directional beam path of predetermined length and an optical focus on said path, (b) locating a lasing means on said path at a position spaced from said focus, (c) periodically energizing said lasing means at a frequency coordinated with said path length such -that pulsed optical energy resonating in said cavity is amplified on passage through said lasing means, and (d) propelling a target into said cavity at said optical focus after a number of passes through said lasing means selected to amplify said energy to a desired level.

16.

9.

A method of operating a free electron laser in a pulsed mode comprising the steps of: (a) providing an optical resonant cavity consisting essentially of high reflectivity mirrors and including opposed concave mirrors having a common optical focus, (b) providing a free electron laser in said cavity at a position spaced from said focus and in an orientation to emit radiation for resonance in said cavity, (c) periodically directing a stream of relativistic electrons through said laser so as to stimulate emission of pulsed radiation for resonance in said cavity at a repetition frequency coordinated with the resonance frequency of said cavity such that said pulsed emissions are cumulatively additive in said laser, and (d) locating a target at said optical focus when said cumulatively additive emissions have reached a desired level.

17.